Method for dispensing fluid materials

ABSTRACT

Fluid to be dispensed is delivered under pressure to a dispensing nozzle by way of an infinitely variable valve which is disposed in sufficiently close proximity to the nozzle that very little fluid pressure drop takes place in the region between the valve and the nozzle. A parameter correlated to the rate of flow of fluid discharged from the nozzle is sensed between the valve and the nozzle to generate a flow rate signal from which a control signal is derived by comparing the flow rate signal with a signal representing a desired rate of flow. Where the nozzle is to be moved relative a workpiece for dispensing fluid material thereon, the latter signal may be derived from a signal correlated to the speed of relative movement between the nozzle and the workpiece such as a tool speed signal from the robot.

FIELD OF THE INVENTION

The present invention relates to a system for dispensing fluids. Moreparticularly, the invention relates to an apparatus and method fordispensing viscous fluid materials such as lubricants, sealants andadhesives onto a workpiece at a controlled rate of flow which can beadjusted to compensate for changes in the relative speed between thedispenser and the workpiece.

BACKGROUND OF THE DISCLOSURE

When dispensing viscous fluids such as certain lubricants, adhesivessealants and the like, it is often necessary to apply the material tothe surface of a workpiece in a bead containing a desired amount ofmaterial per unit length. In high production processes or where the beadof material must be positioned with accuracy, robot arms are often usedto apply the material by rapidly guiding a dispensing nozzle in aprogrammed pattern over the surface of the workpiece. Depending on theapplication, the fluid being dispensed may either be projected somedistance from the nozzle in a high velocity stream or extruded from thenozzle at lower velocity with the nozzle located closer to theworkpiece. In either case, the amount of material applied per unit oflineal distance along the bead will vary according to both the flow rateof material discharged from the dispensing nozzle and the speed of thenozzle with respect to the workpiece.

For example, in the automotive industry it is necessary to apply auniform bead of sealant around the periphery of the inside surface ofautomobile doors before joining the inside panel to the door. Alonglong, straight portions of the pattern, a robot arm can move the nozzlequickly. However, where the desired bead pattern changes directionabruptly, such as around the corners of a door panel, the robot arm mustbe slowed down to achieve a required bead positioning accuracy. It canbe appreciated that if the flow rate of the dispensed fluid material isheld fixed, the amount of material in the applied bead will increase asthe robot arm is decelerated to negotiate changes in direction and willdecrease as the robot arm is accelerated.

In the prior art, one attempt to deal with this problem has been toapply a toolspeed signal emanating from the robot controller to avoltage-controlled D.C. motor drive to control the speed of a ball screwmechanism driving the plunger of a shot pump filled with fluid. The shotpump is connected to the dispensing nozzle on the robot arm by way of alength of flexible hose. The toolspeed signal applied to the D.C. drivevaries with the speed of the nozzle relative to the workpiece. As therate of travel of the shot pump plunger changes, so too does the flowrate from the nozzle. Thus, the rate at which fluid is dispensed iscontrolled in open-loop fashion according to the speed of the nozzle.

Such a system suffers a number of deficiencies. First, it is inherentlyslow to respond. Therefore, only limited control of bead size ispossible. In addition to the delays associated with the response of theD.C. drive and mechanical system driving the plunger, the flexible hoseconnected between the shot pump and the nozzle carried by the robot armintroduces significant response lag into the system. With a hose only 10feet long, and depending on supply pressure and the characteristics ofthe fluid being dispensed, it may take a second or more for a change inpressure at the shot pump to be reflected in a corresponding change inflow at the nozzle. Thus, very precise control of bead size is difficultparticularly during rapid changes in the speed of the robot arm. Inaddition to these performance limitations, such systems have otherpractical disadvantages. The shot pump itself should be capable ofholding at least as much material as required to be applied to an entireworkpiece. Accordingly, the pump and its associated mechanical drive aretoo bulky and massive to be mounted on the robot arm with the dispensingnozzle. The mechanical components and D.C. drive controls together mayweigh up to several hundred pounds. Further, such a system is expensiveto maintain and occupies a significant amount of production floor space.

Another type of system known in the prior art uses a more compactdispenser having a motor driven metering valve which receives acontinuous supply of material by way of a flexible hose. The dispenseris mounted on the robot arm and includes a servomotor or stepper motorwhich controls the metering valve to adjust the flow in accordance withthe speed of the dispensing nozzle as indicated by a toolspeed signalemanating from the robot. Closed-loop control of flow is effected by afeedback signal indicative of material flow deriving, at some point inthe system remote from the dispensing nozzle. This feedback signal maybe derived by sensing the displacement of the supply pump using an LVDTor potentiometer connected to the crosshead of the pump or by using apositive displacement flowmeter connected in line with the flexible hosewhich feeds the dispenser. In addition to this main control loop, such asystem can incorporate a pressure sensor at the nozzle of the dispenserto shut off under specified conditions as described in European patentapplication No. 85-104,127.7. This reference discloses the use of one ormore pressure sensors located in the wall of the dispensing nozzle toderive a pair of signals, one of which is used to indicate the presenceof bubbles, the other of which indicates the flow of the liquid. Thepatent states that the latter signal can be derived for example from apair of contacts connected to an elastic pressure-transmitting elementwhich keeps the contacts closed as long as the pressure at the nozzleexceeds a certain value. In the event a clog develops in the flowchannel, the flow signal can be used to initiate a shutdown of thesystem or provide an indication. Similar action can be taken should abubble be sensed at the nozzle.

This type of system also has significant performance limitations. Eventhough the material being dispensed is metered by a dispenser mounted onthe robot arm rather than from a remote metering device such as the shotpump system described above, the response time of the system is stillrelatively slow. As a consequence, the ability of the system to controlbead size is limited, especially during rapid changes in the relativespeed between the dispenser nozzle and the workpiece.

SUMMARY OF THE INVENTION

It is an objective of the invention to provide a system for dispensingviscous fluid materials having improved speed of response to permit morerapid and precise control of the flow of material being dispensed.

It is a further objective of the invention to provide such a dispensingsystem which is relatively compact and light weight as to be well suitedfor use with robots programmed to define a desired pattern according towhich a bead of material is to be applied to a workpiece.

It is a further objective of the invention to provide a dispensingsystem capable of precisely controlling the amount of material appliedto a workpiece per unit of lineal distance along a bead pattern despiterapid changes in the relative speed between the robot and the workpiece.

It is yet a further objective of the invention to provide such a fluiddispensing system which provides for linearizing the flow response ofthe system by accounting for the dynamic flow characteristics of thefluid as it is dispensed.

It is a still further objective of the invention to provide such a fluiddispensing system which periodically corrects for changes in theintrinsic viscosity of the fluid being dispensed in order to dispense adesired amount of material to each workpiece in a lot.

It is yet another object of the invention to provide an apparatus fordispensing fluids which provides for selectively locating the angularorientation of the fluid material supply hose to avoid interfering withfree movement of the dispenser.

To these ends, a preferred embodiment of the invention includes adispenser for viscous fluids having a servo actuator comprising anelectro-pneumatic servovalve which operates a double-acting pistonactuator. The servo actuator in turn drives a variable fluid meteringvalve. The dispenser includes a discharge nozzle located downstream ofand in close proximity to the fluid metering valve. A pressure sensordisposed at the nozzle and downstream of the needle valve generates apressure signal which is correlated to the instantaneous flow rate ofthe dispensed fluid.

Continuous precise control over flow is achieved utilizing the dispenserin a closed-loop control system whereby the electro-pneumatic servovalveis driven by a control current derived in accordance with the differencebetween the pressure signal and a driving signal representing a desiredflow. In robotic applications, the driving signal is preferably relatedto a toolspeed signal emanating from the robot carrying the dispenser sothat the control current will vary as required to maintain a uniformbead even during relatively rapid changes in the relative speed betweenthe dispenser and the workpiece onto which material is dispensed.

Advantageously, such a system includes means for generating a "pressureoverrange" signal when the pressure in the nozzle exceeds apredetermined value as may occur for example should the nozzle becomeclogged. Also provided are means for generating a "valve overrange"signal when the valve is fully opened and cannot open further. Such asignal is useful for determining that an insufficient amount of materialmay be being dispensed.

Another preferred embodiment of the invention includes an intelligentcontroller which monitors the amount of material being dispensed andcompares it to a desired set point amount. If a deviation is sensed, themagnitude of the setpoint signal is periodically readjusted to zero outthe difference, thereby compensating for changes in the intrinsicviscosity of the fluid. As used herein, the term "intrinsic" refers tochanges in viscosity caused by phenomena other than shear effects. Forexample, intrinsic viscosity changes include variations due totemperature changes. Preferably, the intelligent controller isprogrammed to linearize the flow response of the dispenser to thetoolspeed signal emanating from the robot thereby compensating forpressure flow non-linearities introduced by non-newtonian viscositycharacteristics in the fluid being dispensed.

A preferred dispenser assembly includes a frame securable to a robottool mounting face. One side of the frame supports the servo actuatorwhile the opposite side carries the metering valve assembly whichincludes the pressure sensor. According to the invention, the meteringvalve assembly is secured to the frame in a rotatably adjustable mannerso that the material supply hose may be routed to avoid interfering withfree movement of the dispenser.

These and other advantages will be readily apparent from the followingdetailed description of a preferred embodiment of the invention and fromthe accompanying drawings wherein like reference numerals designate likeitems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view illustrating a preferredembodiment of a dispensing apparatus constructed according to theinvention.

FIG. 2 is a block diagram illustrating a preferred embodiment of asystem for dispensing fluid materials according to the invention.

FIG. 3 is a block diagram illustrating a portion of a second preferredembodiment of a system for dispensing fluid materials according to theinvention.

FIG. 4 is a flow chart illustrating the operation of the embodiment ofFIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 a preferred embodiment of a dispensing gun 10constructed according to the invention is shown. Gun 10 includes aC-shaped frame 11 having a mounting plate 12 adapted to be secured tothe tool mounting face 13 of a robot arm by means of one or more capscrews 14 and alignment pins 15. Frame 10 is preferably constructed of arigid light weight material such as aluminum alloy and further includes,extending outwardly from mounting plate 12, an upper portion 16, and anopposed lower portion 17. The upper portion 16 of frame 11 carries aservo actuator 20 which may consist of any of a number of types ofcompact, light weight linear actuators offering rapid response.Preferably, actuator 20 comprises a double-acting air cylinder 22 havinga piston rod 23 whose degree of extension is controlled by anelectrically actuated pneumatic servovalve 24 disposed atop air cylinder22. The lower portion 17 of frame 11 carries a metering valve assembly26 having a needle valve 27 located between a fluid inlet 28 and adispensing nozzle 29 which includes a nozzle end 30 having a nozzleinlet 29a and a nozzle outlet 31. Valve 27 includes a valve inlet 27aand a valve outlet 27b. For best control, needle valve 27 is located asclose to nozzle 29 as practical and includes a valve stem 32 having agenerally conical end 33 and a valve seat 34. Valve stem 32 is connectedto piston rod 23 so that the position of its conical end 33 relative tovalve seat 34 and hence, the flow rate of fluid discharged from nozzle29 is controlled in accordance with the electrical input ofelectro-pneumatic servovalve 24. A transducer 36 located just downstreamof needle valve 27 generates an electrical signal 37 correlated to therate of flow of fluid discharged from nozzle 29. As will be described infurther detail below, signal 37 is preferably used as a feedback signalto control the rate of flow of fluid dispensed from nozzle 29 inaccordance with a desired driving signal. In robotic applications, thedriving signal can vary with the relative speed between nozzle 29 andthe workpiece 39 to accurately control the amount of fluid per unitlength contained in the bead deposited on the surface of the workpiece39.

Linear actuator 20 may incorporate any of a number of suitable types offast responding electrically actuated servovalves including jet-pipe,nozzle and flapper or spool types. The details of the construction ofactuator 20 are within the purview of those skilled in the art, andaccordingly, do not constitute the claimed invention. In the preferredembodiment illustrated in FIG. 1, actuator 20 comprises a jet-pipeelectro-pneumatic servovalve 24 which operates a double-acting aircylinder 22. Servovalve 24 includes a housing 42 which supports athreaded, electrical connector 43 secured thereto by screws 44. Wired toconnector 43 by way of leads 45 are a pair of series-connected coils 46surrounding opposing ends 49 of an armature 50 which is mounted to pivotabout pivot point 51. A hollow, inverted U-shaped jet pipe 52 has oneleg connectable to a regulated air supply of about 100 PSI nominalpressure through a threaded inlet 53 in air cylinder 22 by way of filter54. The opposite leg of jet-pipe 52 is secured near its center toarmature 50 so that when armature 50 is pivoted clockwise by energizingcoils 46 in one polarity, the flow emanating from jet pipe 52 isdiverted toward a first port 60. Similarly, when coils 46 are energizedin the opposite polarity, armature 50 pivots counter-clockwise to directthe flow from jet pipe 52 toward a second port 61 of air cylinder 22. Ineither polarity, the degree of the deflection of jet pipe 52 and hence,the pressure in ports 60 and 61 is proportional to the magnitude of thecurrent flowing in coils 46. Armature 50 is spring centered andmagnetically biased such that when coils 46 are in a de-energized state,jet pipe 52 is centered in a neutral position as shown so that thepressures in ports 60 and 61 tend to be equally balanced. Magnetic biasis provided by a pair of permanent magnets 63 each of which communicatewith the armature field by way of a flux across air gaps 65. This fluxis conducted to gaps 65 by way of four magnetically permeable members 66arranged as shown.

Double acting air cylinder 22 includes an aluminum alloy cylinder body70, the end of which is received in a hole 71 in the upper portion 16 offrame 11. A flange 72 is used to secure the body 70 of air cylinder 22to the upper portion 16 of frame 11 using cap screws 73. Cylinder body70 includes first and second ports 60, 61, threaded air supply inlet 53and filter 54 as well as a cylinder bore 75. Received within bore 75 isa piston 76 provided with a pair of seals 78 as well as piston rod 23which extends axially from bore. The portion of bore 75 located abovepiston 76 communicates with first port 60 while the portion beneathpiston 76 is connected to second port 61. The force with which piston 76drives needle valve 27 depends upon the differential pressure betweenports 60 and 61 which, as explained above, is determined by thedeflection of jet pipe 50 due to the current flowing in coils 46. Pistonis retained within cylinder bore 75 by a cap 80 through which passespiston rod 23. To prevent air leakage cap 80 is provided with a seal 81in the area of piston rod 23 and an external O-ring seal 82 between theouter circumference of cap 80 and the surface of cylinder bore 75. Cap80 is itself retained in the end of cylinder bore 75 by a snap-ring 83.

Metering valve assembly 26 includes a rigid, non-resilient valve body 85constructed as shown in FIG. 1 preferably of metal. The lower end ofvalve body 85 includes a passage 84 whose lower end is threaded toaccept the flow restricting nozzle 29 of a desired configuration havinga discharge outlet 31. Passage 84 is intersected by one or more radialthreaded holes, one of which receives transducer 36 and the others ofwhich are sealed by means of plugs 90. Located immediately upstream ofpassage 84 and as closely adjacent thereto as practicable, valve body 85houses needle valve 27. For long life, both valve stem 32 and valve seat34 are preferably fabricated of a hard material such as sinteredtungsten carbide. A fluid supply inlet 28 enters valve body 85 upstreamof needle valve 27. Inlet 28 is threaded so that a hose can be attachedto supply under pressure the fluid material to be dispensed.

Valve body 85 threads onto the lower end of a bonnet 97 and is sealedwith respect thereto by means of an O-ring seal 98. Bonnet 97 includesan internal packing gland 99 which holds a plurality of annular PTFEpacking seals 100. Seals 100 are retained in sealing but nonbindingcompression about valve stem 32 by means of any adjustable gland nut101. To attach metering valve assembly 26 to frame 11, bonnet 97 isthreadably received by the extending lower portion 17 of frame 11 andsecured thereto at a desired angular orientation by means of a locknut102. Metering valve assembly 26 is connected to actuator 20 by means ofa coupling 105 which is fixedly attached to the upper end of valve stem32 and threaded onto the lower end of piston rod 23 and held in place bya second locknut 106.

Transducer 36 may comprise any suitable transducer capable of generatinga signal 37 indicative of the rate of flow of the fluid dispensed fromnozzle 30. Preferably, transducer 36 is a strain gauge pressuretransducer operably disposed to sense the instantaneous fluid pressureat a location inside passage 84 immediately downstream of needle valve27. One pressure transducer suitable for this purpose is model A205manufactured by Sensotec of Columbus, Ohio. The flow of a viscousnewtonian fluid at low Reynolds numbers is substantially linearlyproportional to the pressure drop across a nozzle or tubular restrictorplaced in the flow path. It can be appreciated that a pressuretransducer 36 located as described will sense the pressure drop acrossnozzle 29. This is so because the outlet 31 of nozzle 29 is atatmospheric pressure and there is very little pressure drop acrosspassage 84 in relation to the pressure drop across nozzle 30. Thus,transducer 36 generates a pressure signal 37 which represents theinstantaneous rate of flow from outlet 31. As previously noted, due tothe proximity of needle valve 27 this flow is closely correlated to theflow through needle valve 27. Since flow rate is sensed by pressuretransducer 37 and controlled by needle valve 27 both in close proximityto nozzle 29, precise control over flow rate, and hence, the amount offluid per unit length deposited by gun 10 on workpiece 39 can beachieved by connecting dispensing gun 10 to form a fast respondingclosed-loop servo control system as described now with particularreference to FIG. 2.

Dispensing gun 10 is carried by the tool mounting surface 13 of a robothaving a controller (not shown) programmed to guide nozzle 29 over thesurface of a workpiece to dispense a bead of fluid thereon in a desiredpattern. The metering valve assembly 26 of gun 10 communicates at itsfluid inlet 28 with a continuous pressurized supply of fluid. Transducer36 continuously senses the pressure drop across nozzle 29 to generate apressure signal 37 correlated to the rate of flow of fluid dischargedfrom the outlet 31 of nozzle end 30. Signal 37 is received and amplifiedby a preamp 110 which generates an output signal 111 appearing at theminus input 112 of a summing junction 113 as well as at a first input114 of a comparator 115 whose second input 116 receives a fixed,selectable voltage reference, VREF1 and whose output 117 generates adigital PRESSURE OVERRANGE signal 118 which is received by the robotcontroller. If the magnitude of output signal 111 exceeds VREF1, digitalPRESSURE OVERRANGE signal assumes a logical 1 value. This can occur forexample if needle valve 27 opens too far. In such event, the robotcontroller can be programmed to present a fault indication, shut downthe system or take other appropriate action. Summing junction 113 alsoincludes a plus input 119 which receives a driving signal 122. In theembodiment of FIG. 2, driving signal 122 is generated by an amplifier127 in accordance with a toolspeed signal 128 from the robot. Toolspeedsignal 128 is an analog voltage signal available from the robotcontroller which varies according to the speed of travel of gun 10relative to workpiece 39. Through the robot controller, the gain ofsignal 128 can be adjusted by way of a toolspeed multiplier selected toprovide a desired flow rate as a function of speed of travel. Amplifier127 is an operational amplifier whose gain is selected to properly scaletoolspeed signal 128 so that driving voltage 122 will be within a rangecompatible with the rest of the circuit. Amplifier 127 is preferablyconnected as a precision limiter such that for inputs between zero voltsand an adjustable threshold voltage, the voltage of driving signalexecutes a decisive step in a direction proper to close needle valve 27.Typically, the threshold voltage would be adjusted so that whentoolspeed signal 128 is about 50 mV or less, needle valve 27 is drivenpositively closed. This prevents needle valve 27 from leaking byproviding a negative bias current to servovalve 24, effective to driveneedle valve 27 positively closed at times when toolspeed signal 128 isnot present or quite small. Summing junction 113 produces an analogerror signal 130 whose magnitude and polarity is equal to the algebraicdifference between the output signal 111 of preamp 110 and drivingsignal 122. Error signal 130 is received by an amplifier 131 whose gainis adjusted for optimum system stiffness. The output signal 132 fromamplifier 131 is received by a lead/lag compensation network 134designed and adjusted according to standard control technique tostabilize closed-loop system response and maximize response speed withminimum overshoot. A second summing junction 135 then adds a dithersignal 136 from a dither generator 137 to the output signal 138 oflead/lag network 134. Dither signal 136 is an A.C. signal whosemagnitude preferably several percent of the fullscale value of signal138. Dither signal 136 improves system resolution by overcoming staticfriction effects. Dither signal 136 accomplishes this by causing aircylinder 22 to oscillate very slightly during system operation, as iscommonly practiced in the art. Summing junction 135 provides an analogvoltage signal 139 whose magnitude and polarity is determined by thealgebraic sum of signal 138 and dither signal 136. Signal 139 isreceived by a current driver 140 as well as by the first input 141 of acomparator 142 whose second input 143 receives a fixed, selectablevoltage reference, VREF2 and whose output 144 generates a digital VALVEOVERRANGE SIGNAL 145. In the event the magnitude of signal 139 exceedsVREF2, digital VALVE OVERRANGE signal assumes a logical 1 state. Such acondition may arise for example if the supply of fluid to dispensing gun10 is cutoff or if supply pressure is inadequate to meet the demandimposed by driving signal 122. Like PRESSURE OVERRANGE signal 118, VALVEOVERRANGE signal 145 is directed to the robot controller which may beprogrammed to generate a fault indication, shut the system down orotherwise initiate corrective action.

Current driver 140 generates an analog control current signal 146 whichis applied to the coils 46 of servovalve 24. This causes jet pipe 52 tobe diverted toward first port 60 or second port 61, depending on themagnitude and direction of control current signal 146, to move thepiston 76 of air cylinder 22 either downward or upward, respectively.Downward movement of piston 76 tends to close needle valve 27 ofmetering valve assembly 26 thereby reducing the flow of fluid whileupward movement of piston 76 tends to open needle valve 27 therebyincreasing the flow of fluid.

In operation, the system functions as a closed loop servo systemresponsive to the pressure drop across nozzle 29 as sensed by pressuretransducer 36. With needle valve 27 initially closed, no flow occurs andthe pressure drop across nozzle 29 is zero. Assuming toolspeed signal128 is less than the threshold voltage associated with amplifier 127,amplifier 127 generates a driving signal 122 of the proper polarity andof sufficient magnitude to generate a control current 146 to deflect jetpipe 52 toward first port 60. This holds piston 76 down so that needlevalve 27 is held closed under force thereby preventing leakage. Thiscondition is maintained until toolspeed signal 128 rises above thethreshold voltage of amplifier 127 indicating that flow should commence.When this occurs, driving signal reverses polarity. Since there isinitially no flow, pressure signal 137 is at its zero value.Accordingly, an error signal 130 whose magnitude is determined by thedifference between pressure signal 37 and driving signal 122 will causea control current 146 to be applied to coils 46 in such a polarity as tocause jet pipe 52 to deflect toward second port 61. In response, piston76 moves upward causing needle valve 27 to open by lifting the conicalend of valve stem 32 away from valve seat 34. As the pressure signal 37generated by pressure transducer 36 increases error signal 130 andcontrol current 146 both decrease and jet pipe 52 moves toward its nullposition. As the pressure drop across nozzle 29 approaches a valuecorresponding to a desired flow rate jet pipe 52 causes needle valve 52to remain open by an amount just sufficient to maintain the pressuredrop across nozzle 29 at that value.

In some dispensing applications, the flow characteristics of the fluidsupplied to dispensing gun 10 may be subject to change over time. Forexample if gun 10 is supplied fluid from a drum, the viscosity of thefluid can vary with changes in temperature as the drum sits in a warmproduction area after having been moved from a cold warehouse. Viscositymay also vary from one drum of fluid to the next or from the top of agiven drum to the bottom. Without some means for compensating for suchchanges, the amount of material dispensed onto a workpiece 39 would besubject to undesirable variations. Also, when dispensing non-newtonianfluids, the overall instantaneous viscosity of the fluid varies withshear rate in a non-linear fashion. Thus, absent correction, shearinduced by the geometry of nozzle 29 will result in a non-linear flowrate versus pressure signal 37 flow characteristic. This in turn wouldrender the flow rate versus applied toolspeed signal 128 responsenonlinear. According to the invention, these problems are effectivelyaddressed by deriving driving signal 122 in an alternate fashion asdescribed now with additional reference to FIGS. 3 and 4.

FIG. 3 illustrates a second preferred embodiment of the invention whichis similar to the embodiment described above except for the manner inwhich driving signal 122 is generated. As illustrated in FIG. 3, thesystem of FIG. 2 is modified by adding a positive displacement flowmeter 150 to the fluid supply line connected to the inlet 28 ofdispensing gun 10. While it is desirable to locate flow meter 150 asclose to gun 10 as possible it is not required to be mounted with thegun 10 on the robot arm. Flow meter 150 includes an incremental encoder152 which produces an electrical output signal 153 comprising a seriesof pulses 155. Each pulse 155 represents a predetermined volume offluid. Signal 153 is input to a pulse counter 156 which counts pulses155 and is resettable to zero by a reset signal 158 which is generatedby a microprocessor based controller 160 which, if desired may be partof the robot controller (not shown). However, to provide maximum systemfrequency response, controller 160 should run at high speed and ispreferably dedicated principally to performing the operations describedbelow. In addition to a microprocessor and associated hardware,controller 160 includes all necessary program and data memory as well asan analog to digital converter (A/D) 163 which receives the toolspeedsignal 128 from the robot controller. Pulse counter 156 outputs itspulse count 165 to controller 160. Controller 160 also receives from therobot controller (not shown), a digital cycle status signal 168 and adigital job status signal 170. Cycle status signal assumes a logical 1value whenever dispensing gun 10 should be operating. Job status signal170 assumes a logical 1 valve when a production run is at an end.Controller 160 also communicates by way of an interface 172 with aninput/output device 175 such as a keyboard terminal from which controlcommands and setpoint data are entered. Controller 160 also communicatesby way of an output 176 with a digital to analog D/A converter 177 whichgenerates an analog signal 178. Signal 178 is received by amplifier 127which operates as described above with reference to FIG. 2. Amplifier127 in turn generates driving signal 122 which is applied to the plusinput 119 of summing junction 113 as described above to generate errorsignal 130. The manner in which driving signal 122 is derived may befurther understood with additional reference now to FIG. 4 whichillustrates the software program stored in controller 160 responsiblefor outputting the required data to D/A converter 177.

The program begins running by clearing all data memory and initializingall variables including a setpoint representing a desired total volumeof fluid to be applied to a single workpiece 39. An appropriate set ofpre-programmed flow linearizing factors (FLFs) are also initialized atthis point. The FLF's are constants which represent factors by whichtoolspeed signal 128 must be multiplied in order to linearize systemflow response such that when a given percentage of the full scale valueof toolspeed signal 128 is applied to summing junction 113, the needlevalve 27 of metering valve assembly 26 is positioned so that the samepercentage of the full scale flow of fluid is discharged from nozzleoutlet 31. FLF's are determined empirically from a measured curve ofactual flow from outlet 31 of nozzle 30 versus voltage applied at input119 of summing junction 113. Since the actual flow curve may varydepending on the geometry of needle valve 27 and nozzle 29 includingnozzle end 30 as well as the flow characteristics of the particular typeof fluid being dispensed and the supply pressure, the program loads aseries of FLF's appropriate to account for a particular set of theseconditions.

The program also sets a flow compensation factor (FCF) to an arbitrarilyselected initial value. The FCF is a variable which compensates forchanges in the flow characteristics which occur over time such aschanges in intrinsic viscosity due to changes in temperature or otherfactors as discussed earlier. The FCF is recomputed once each job cyclethat is, once per dispensing operation on a given workpiece 39. The FCFis defined as a factor by which the linearized toolspeed signal must bemultiplied so that the total volume of fluid dispensed onto a workpiece39 is substantially equal to the selected setpoint. Deviation fromsetpoint cannot be determined at the beginning of the first job cyclebecause there is no basis for comparison. Accordingly, FCF is preferablyinitialized at unity. The manner in which FCF is recomputed will bedescribed below.

During initialization, the program resets pulse counter 156 to zero byoutputting an appropriate reset signal 158 from controller 160 tocounter 158. Next, the program causes controller 160 to read the totalpulse count 165. The value of pulse count 165 represents the totalvolume of fluid dispensed during the previous job cycle. If pulse countis not zero, as will be the case except prior to the first job cycle,the program recomputes the flow compensation factor FCF as a quotientwhose dividend is equal to the setpoint and whose divisor is equal tototal pulse count 165. After the FCF is recomputed counter 156 is againreset in the manner described above. If pulse count 165 is equal tozero, as it will be at the beginning of the first job cycle, the FCFremains at its initialized valve.

Next, the program enters a loop in which it waits for the robotcontroller signal that a job cycle is in progress. In the wait loop, theprogram continuously reads cycle status signal 168 and tests todetermine whether it has assumed a logical 1 value. If not, the programstays in the loop. By changing status signal 168 from a logical zerovalue to a logical 1 value, the robot controller indicates thatdispensing should commence. At that point, the program directscontroller 160 to read the digital value 180 representing the magnitudeof toolspeed signal 128 from the output of A/D converter 163. Based onthe magnitude of the digital value, the program selects from a look-uptable the corresponding flow linearizing factor FLF from the set of FLFvalues loaded during initialization. Digital value 180 is thenmultiplied by the selected FLF value to yield a linearized toolspeedvalue 181. To adjust driving signal 122 so that the actual volume offluid to be dispensed during the job cycle conforms to the setpointdespite changes in the flow characteristics of the fluid, such aschanges in viscosity, the program next causes the linearized toolspeedvalue 181 to be multiplied by the flow compensation factor FCF to yielda corrected digital value 182 which is then output to D/A converter 177whose output 178 is fed to amplifier 127 to generate driving signal 122.

Next, the program again reads cycle status signal 168 to determinewhether dispensing should continue. If not, job status signal 168 willnot be a logical 1 value, indicating the present cycle has ended. Inthat case the program causes controller 160 to read job status signal170 emanating from the robot controller. If job status signal 170 is nota logical 1 value, this indicates that the last workpiece 39 in a givenproduction lot has been finished and the program is stopped. If theproduction run is not complete, job status signal 170 will remain at alogical 1 value and the program will loop back to the point at whichpulse count 165 is read. Although the program described recomputes aflow compensation factor once per job cycle, it should be noted thatsuch periodic adjustments can be made more or less frequently dependingon how rapidly the flow characteristics of the dispensed fluid can beexpected to undergo significant change.

The advantages realized by the invention are numerous. Most notably, thedispensing systems described provide rapid and precise control of fluidflow rate. Such systems have been found to have an upper 3 dB frequencyresponse cutoff point exceeding 10 hertz.

While the dispensing gun 10 can be directed by any desired meansincluding manually the invention is particularly well adapted for usewith robots. Dispensing gun 10 is light weight, compact and easy tomaintain. Further, the dispensing systems of the invention provide forautomatic flow rate adjustment in accordance with the relative speedbetween the dispensing gun 10 on the robot arm and the workpiece.

Thus, the invention permits close control over the volume per unitlength of the dispensed bead of fluid even during rapid acceleration anddeceleration as normally occurs as the robot arm changes its directionof movement. The invention also provides means for periodicallycompensating for perturbations in the flow characteristic of the fluidbeing dispensed to insure that the volume of fluid dispensed alwaysconforms closely with a desired setpoint.

While the above descriptions constitute preferred embodiments of theapparatus and method of the invention, it is to be understood that theinvention is not limited thereby and that in light of the presentdisclosure of the invention various alternative embodiments will beapparent to persons skilled in the art. Accordingly, it is to beunderstood that changes can be made to the embodiments described withoutdeparting from the full legal scope of the invention which isparticularly pointed out and distinctly claimed in the claims set forthbelow.

What is claimed is:
 1. A method of dispensing fluid material, saidmethod comprising the steps of:(a) delivering the fluid under pressureto a valve having a valve inlet and a valve outlet and conducting thefluid from said valve to a nozzle having a nozzle inlet and a nozzleoutlet from which said fluid is dispensed, said valve outlet beingcoupled to said nozzle inlet, said valve outlet and said nozzle inletbeing disposed in sufficiently close proximity to one another that verylittle fluid pressure drop occurs between said valve outlet and saidnozzle inlet; (b) sensing, at a location between said valve outlet andsaid nozzle inlet a parameter other than the position of a flowrestricting element, said parameter being correlated to the rate of flowof the fluid discharged from said nozzle outlet and generating acorresponding flow rate signal; (c) generating a control signal from atleast said flow rate signal, and (d) operating said valve in accordancewith said control signal to substantially infinitely variably modulatethe flow of fluid dispensed from said nozzle outlet when said valve isat least partially open and to positively cut off flow when said valveis closed.
 2. The method of claim 1 wherein said parameter is thepressure of the fluid at said location.
 3. A method of dispensing fluidmaterial onto a workpiece, said method comprising the steps of:(a)delivering the fluid under pressure to a valve having a valve inlet anda valve outlet and conducting the fluid from said valve to a nozzlehaving a nozzle inlet and a nozzle outlet from which said fluid isdispensed, said valve outlet being coupled to said nozzle inlet, saidvalve outlet and said nozzle inlet being disposed in sufficiently closeproximity to one another that very little fluid pressure drop occursbetween said valve outlet and said nozzle inlet; (b) sensing, at alocation between said valve outlet and said nozzle inlet a parametercorrelated to the rate of flow of the fluid discharged from said outletand generating a corresponding flow rate signal; (c) generating acontrol signal from at least said flow rate signal and a signalcorrelated to the speed of relative movement between said nozzle outletand the workpiece, and (d) operating said valve in accordance with saidcontrol signal to substantially infinitely variably modulate the flow offluid dispensed from said nozzle outlet when said valve is at leastpartially open and to positively cut off flow when said valve is closed.4. The method of claim 3 wherein said parameter is the pressure of thefluid.
 5. The method of claim 4 further comprising the step ofgenerating a pressure overrange signal in the event that said pressureexceeds a predetermined limit.
 6. The method of claim 3 furthercomprising the step of linearizing said flow of the fluid with respectto said signal correlated to said speed of movement between theworkpiece and said nozzle.